In recent years, the traffic of mobile communication has been increasing as mobile phones have become more sophisticated, and it is desired to implement further higher speed wireless communication in a limited frequency band. One of methods to implement this is a multiple-input multiple-output (MIMO) transmission technology (for example, JP 2014-131181 A). In MIMO transmission, a plurality of signal sequences are transmitted from a plurality of transmission antennas at the same time and at the same frequency, and spatial multiplexing is performed. By utilizing the multipath environment between the respective transmission antennas and reception antennas, signal processing is performed on a reception machine side to separate and decode each signal. As a result, without expanding a frequency bandwidth to be used, the capacity of the transmission path can be increased in proportion to the number of transmission and reception antennas. In other words, the frequency utilization efficiency can be improved. In mobile communication, which is not a line-of-sight communication system, a reception machine receives a plurality of signals by reflection from the surroundings. This propagation environment varies with time due to movement of a moving object using the system or an object causing scattering in the surroundings. In the case of using a plurality of antennas at such intervals that the above propagation environment can be regarded as independent, this is equivalent to a situation where a plurality of transmission paths exists. Thus, a plurality of independent signals corresponding to the number of antennas can be transmitted in the same frequency band.
In the situation where radio frequencies of the microwave band are tight along with the increasing traffic of mobile communication, it is required to increase the transmission capacity using high frequency bands such as the millimeter wave band. Comparing millimeter wave band communication to microwave band communication, there is an advantage in millimeter wave band communication that the millimeter wave band communication is suitable for increasing the capacity since signals in relatively wider bands can be handled; however, there are also disadvantages that it is difficult to perform large-power amplification and that the communication distance becomes shorter since the propagation loss is large. As a countermeasure against the above, there is a beamforming technique that performs spatial synthesis using a plurality of antenna elements. This is a technique of controlling the phase of transmission signals output from a plurality of antenna elements on the transmission side such that in-phase synthesis that maximized the power is performed in a reception machine. As the number of antenna elements increases, a directivity of the main lobe becomes sharper with side lobes in other directions attenuating. Therefore, the beamforming technique attracts attention also as an interference suppression technique. As an interval between antenna elements, λ/2 (λ: wavelength) is suitable. In the millimeter wave band of short wavelength, a relatively small antenna unit can be implemented.